多晶体材料微米尺度下弹塑性行为的实验表征技术研究

The deformations of polycrystalline materials usually feature strong heterogeneity at micrometer scale, resulting in discrepancies between local stress/strain and macroscopic stress/strain. However, due to the relative immaturity of experimental techniques available at micrometer scale, the elasto-plastic behavior at such scale has not been thoroughly understood. The present project aims at establishing an experimental framework that characterizes the micrometer-scale elasto-plastic behavior of polycrystalline materials. In this framework, stress and geometrically necessary dislocation (GND) density will be measured by Laue microdiffraction, in which the experimental parameters are optimized by virtual experiments. Thanks to the recently developed "enhanced Laue-DIC method", we can anticipate measurements with better accuracy and positional precision than precedented ones. The stress and GND density measurements from monocrystalline specimens will be crosschecked with theoretical solutions to evaluate their accuracy. For polycrystalline specimens, the diffraction measurements will be coupled to the strain field measured by digital image correlation method via fiducial markers deposited on the surface of specimen. The purpose of such coupling is producing stress-strain curves of the diffracting regions and thereby examining the current dislocation density based crystal plasticity models. The aforementioned framework will be conducive to elucidate the interactions between micrometer-scale elasto-plastic deformation and GNDs in polycrystalline materials, and lay the experimental foundation for the development of crystal plasticity model.

多晶体材料的变形通常在微米尺度下表现出强烈的不均匀性，引起材料的局部应力/应变与宏观应力/应变之间的差异。由于微米尺度力学实验技术还相对不成熟,人们尚未充分了解多晶体材料在该尺度下的弹塑性行为。本项目旨在建立一整套表征多晶体材料微米尺度弹塑性行为的实验手段。利用Laue微区衍射技术测量应力和几何必需位错密度，其实验参数将通过虚拟实验优化。最近发展起来的改进型Laue-DIC方法预计会提高测量的准确性和位置精度。测量得到的单晶试样的应力和几何必需位错密度将会和理论解对比评估其准确性。对于多晶试样，衍射测量结果将通过试样表面沉积的标记来和数字图像相干方法测得的应变场耦合，目的是为了得到衍射区域的应力-应变曲线和几何必需位错的演化，并用来检验当前基于位错密度的晶体塑性模型。以上的实验手段有助于阐明多晶体材料微米尺度下弹塑性变形和几何必需位错的相互作用，为晶体塑性模型的发展提供实验支持。

工程多晶体材料（合金、金属基复合材料等）在微米尺度下的局部应力/应变通常和其宏观应力/应变有较大的差异，表现出强烈的不均匀性。这种不均匀性与材料的失效行为高度相关。然而由于微米尺度下的材料实验手段还相对滞后，人们尚未充分了解多晶体材料在该尺度下的弹塑性行为。本项目开发了包括Laue微区衍射、纳米压痕以及电镜环境下原位应变测量等多种手段，以满足研究材料在微米尺度下的弹塑性力学行为的需求：电镜环境下能同时实现组织观测和应变测量的方法，适用于颗粒增强金属基复合材料；Laue微区衍射扫描图案的机器学习处理算法，包括基于非监督式学习的晶粒划分程序和面向衍射图案标定的卷积神经网络；实现了利用纳米压痕实验得到多晶体基体中单个晶粒的正交各向异性弹性参数和晶体塑性参数的方法，并优化了迭代效率。这些方法丰富了材料在微米尺度下的研究手段，为进一步揭示应力/应变的局部不均匀性奠定了基础。